1. Field of the Invention
The present invention concerns a field emission cathode as well as an x-ray tube with such a field emission cathode.
2. Description of the Prior Art
In x-ray tubes, thermionic emitters (advantageously made of tungsten, tantalum or rhenium) are conventionally used to generate the electron beam required for the generation of x-ray radiation. The thermionic emitter is heated to approximately 2,000° C., causing electrons to be thermionically emitted, and the emitted electrons accelerated toward an anode by an electrical potential of approximately 120 kV. X-ray radiation usable for imaging is created when the thermionically generated electrons strike the anode. Such a thermionic emitter is described in DE 27 27 907 C2, for example. Such thermionic emission has the disadvantage that switching of the heating current requires several seconds since the heating of the thermionic emitter is slow.
As an alternative to the generation of free electrons by means of thermionic emission, the possibility exists to generate free electrons by field emission. By applying a voltage, electrons are extracted from a material with a high emission density, for example carbon nanotubes (CNT), and heating of this material is not necessary. The current densities that can be achieved with such a field emitter are typically less than 1 A/cm2, but well below the current densities of a thermionic emitter (with which current densities up to 10 A/cm2 can be realized). The possibility to quickly switch such a field emitter (known as a “cold emitter” due to the fact that a heating is unnecessary, or only a slight heating is required) makes this technology very attractive for x-ray tubes. If the current density is increased to a few A/cm2, the lifespan of the field emitter is limited. In order to increase the lifespan it is known to arrange multiple emitter modules in parallel in order to distribute the total load of the field emitter among them, thus reducing the total load for the individual emitter modules, and thereby increasing the lifespan of the field emitter. The manufacture of such emitter modules is complicated and consequently is expensive. Furthermore, each emitter module must be activated individually. Therefore this concept can only be realized with technical difficulty in rotating anode x-ray tubes.
In order to achieve high field strengths of greater than 1 V/μm for the electron emission, either a high voltage is required or the distance to the anode must be very short. An additional possibility is the use of an extraction grid (gate electrode) between the field emitter and the anode, that is at a positive potential relative to the electron emission layer. Given distances between approximately 100 μm and 1 mm, the aforementioned field strengths can be generated with average voltages in the range of a few kV, which can be handled easily. The extraction grid is composed of thin tungsten wires, for example, with a wire diameter of a few 10 s of μms, and typically exhibits a grid spacing of 100 to 200 μm.
An x-ray tube with a field emission cathode that has a field emitter and an extraction grid is known from the product brochure “Carbon Nano Tube Based Field Emission X-Ray Tubes”, for example. This product information is available at www.xintek.com/products/xray/index.
A rotating anode x-ray tube and a rotary piston x-ray tube that has cold emitters as the electron source are described in DE 10 2005 049 601 A1 and in the corresponding United States Application Publication No. 2007/0086571.
Field emission cathodes with an electron emission made of carbon nanotubes (CNT) are known from U.S. Pat. No. 6,553,096. An extraction grid that is at a positive potential relative to the electron emission is arranged between the field emitter and the anode.
A field emitter with a rod-shaped nanostructure (“nanorods”) is disclosed in United States Application Publication No. 2007/0247048.